1. Field of the Invention
The present invention relates to a rubber composition utilizing a reclaimed rubber obtained by reclamation of a vulcanized rubber such as a waste tire and also relates to a method for producing the same.
2. Description of the Related Arts
Substantial issues on the recycling of industrial wastes arose in response to an increasing consciousness of environmental protection in these days. In such circumstances, a rapid improvement in the recycling efficiency of a rubber product such as a waste tire is desired.
Conventionally, a rubber product such as a waste tire has generally been reused as a kiln fuel for a cement or as a reclaimed rubber, while the reuse as a reclaimed rubber to be recycled again as a rubber product is most desirable.
Some conventional methods for reclaiming a vulcanized rubber are exemplified below.
(1) A reclaiming agent is added to a coarsely pulverized vulcanized rubber and the mixture is heated in an autoclave (for example in a condition of at 200.degree. C. and 14.5 kg/cm.sup.2 for 5 hours) and then refined using a finishing roll, whereby obtaining a reclaimed rubber. This method is referred to as PAN method.
However, a reclaimed rubber obtained by PAN method described above or a re-vulcanized rubber obtained therefrom by means of re-vulcanization of the reclaimed rubber exhibits a low strength when used alone, and thus is a rubber whose application range is extremely limited. As a result, it is difficult to use such reclaimed rubber alone to form a practical rubber product (including re-vulcanized rubber and the like).
Accordingly, it has been attempted that the reclaimed rubber discussed above is added as an additive to a virgin rubber to obtain a rubber molded article for which the consumption of the virgin rubber is reduced.
(2) A method has been known in which a vulcanized rubber is pulverized into a particle whose particle size is several hundreds .mu.m and then added to a non-vulcanized virgin rubber. Also in this case, a rubber molded article for which the consumption of the virgin rubber is reduced can be obtained.
However, even when a reclaimed rubber is added to a virgin rubber, the fracture properties, the abrasion resistance, the curing properties and the like are affected adversely, and furthermore, even when a vulcanizing agent is added subsequently to conduct re-vulcanization, it is still difficult to produce a practical rubber product.
Further, a blend rubber obtained by mixing a pulverized vulcanized rubber with a virgin rubber readily undergoes a destruction occurring initially at an interface between a molecule of the vulcanized rubber and a molecule of the virgin rubber due to an insufficient binding at this interface. Accordingly, it is difficult to obtain an excellent reclaimed rubber of a satisfactory quality by these methods for reusing.
(3) A method for reusing a vulcanized rubber (for example a waste tire) containing a thermoplastic resin fiber such as a rubber waste is disclosed in Japanese Laid-Open Patent Publication No.56-76439. This method utilizes a simple reclaiming process. Thus, a vulcanized rubber powder from, for example, a rubber waste is devulcanized to plasticized material, kneaded together with a sulfur and then kneaded again together with a thermoplastic resin having a rubber-like elasticity with heating at a temperature not lower than a vulcanization temperature, whereby obtaining a material for injection molding.
A method disclosed in Japanese Laid-Open Patent Publication No.56-76439 involves the following problems in addition to the problems involved in PAN method discussed above.
[1] The steps of producing are complicated, time and labor consuming and expensive.
[2] A sulfur should be added for re-vulcanization in order to improve adhesion properties and physical properties.
[3] Since a devulcanized rubber supplemented with a vulcanizing agent and a thermoplastic resin are kneaded with heating at a temperature not lower than the vulcanization temperature, the devulcanized rubber disperses throughout the entire thermoplastic resin phase, while being re-vulcanized. In such a process, micro-dispersion is difficult to be achieved, and a composition as an alloy of a rubber phase and a thermoplastic resin phase can not be obtained.
[4] Although a material for injection molding can be obtained from a vulcanized rubber such as a rubber waste and a thermoplastic resin having a rubber-like elasticity, the resultant material becomes a rubber-like elastomer, whose range of application is limited and whose technological utility is also poor.
(4) When a vulcanized rubber such as a rubber waste containing a thermoplastic resin fiber is reclaimed in PAN method, the standard processing temperature of PAN method which is not higher than the melting point of a the thermoplastic resin allows the shape of the fiber to be kept as it was in the reclaimed rubber and also allows a strainer to be plugged readily. In addition, the fiber engages with each other to form an aggregation when the finishing roll was employed in the condition mentioned above, resulting in the aggregation part in which the fiber is poorly dispersed, which may affect the surface quality and the physical properties of a molded article which is a re-vulcanized material.
Even when a vulcanized rubber such as a rubber waste still containing a thermoplastic resin fiber is treated at a temperature not lower than the melting point of the thermoplastic resin, the molten thermoplastic resin aggregates to form a bulky particle of several hundreds .mu.m or larger since it has not been subjected to a sheer stress. As a result, a part containing this bulky particle serves to deteriorate the surface quality and the physical properties of a molded article which is a re-vulcanized material.
(5) Japanese Patent Application Laid-Open No.5-329842 discloses a method for obtaining a rubber product in which the shape of the fiber contained in the rubber is kept as it was.
A method according to Japanese Laid-Open Patent Publication No. 5-329842 involves a problem similar to that experienced in PAN method. Accordingly, a re-vulcanized material of a rubber obtained can be used only in an article whose surface quality is not taken into account, such as those employed in construction and building works.
The conventional methods described above such as the rubber powder method and PAN method can also be applied to the reuse of a vulcanized rubber such as a rubber waste containing a fiber, if the fiber has previously be removed. For this purpose, however, pulverization by means of, for example, freeze-grinding should be followed by separation by means of such as blow-sieving, resulting in a high cost.